System and method for automatically calibrating transducers in electro-pneumatic freight brake control systems

Abstract

A system and method of calibrating pressure transducers in an electro-pneumatic brake system for a railroad train in which a locomotive microprocessor generates a respective best fit curve for the train brake pipe, supply reservoir and brake cylinder pressures that approximates the actual train pressure therefore. These best fit curves are generated from the pressure transducer readings of these pressures at each car by employing an equation based on a fourth order polynomial. Each car is then provided with a theoretical reference pressure signal from the best fit curve for each of the mentioned brake pipe, supply reservoir and brake cylinder pressures, according to the position of the car in the train. The theoretical reference signal is then compared at each car with the car pressure transducer reading for the respective brake pipe, supply reservoir and brake cylinder pressures to obtain a transducer error correction factor that remains constant through a full range of pressures. The error correction factor can be further calculated on the basis of a linear equation to obtain a variable error correction factor.

I claim: 
     
       1. A system for calibrating pressure transducers in an electro-pneumatic brake control system for a train of railroad cars having pneumatic and electric communication means between the train locomotive and respective cars, each car having in addition to said pneumatic communication means, a supply reservoir connected to said pneumatic communication means and a brake cylinder device connected to said supply reservoir, said calibration system comprising: a. pressure transducer means for providing a feedback signal according to the pressure of at least one of said pneumatic communication means, said supply reservoir and said brake cylinder device at each said car;   b. means for calculating a mathematical best fit curve that closely approximates the actual natural gradient of train pressure for said at least one of said pneumatic communication means, said supply reservoir and said brake cylinder device in accordance with corresponding ones of said transducer feedback signals being connected to said microprocessor means;   c. means for deriving from said best fit curve a theoretical reference pressure value for each said car depending on its location in said train; and   d. means for determining a difference between said theoretical reference value and said transducer feedback signal corresponding thereto for each said car to use as a transducer error correction factor.   
     
     
       2. A transducer calibration system as recited in claim 1, wherein said transducer error correction factor is constant for any fluid pressure of said at least one of said pneumatic communication means, said supply reservoirs and said brake cylinder devices. 
     
     
       3. A transducer calibration system as recited in claim 1, wherein said transducer error correction factor is variable according to the fluid pressure effective at different cars for said at least one of said pneumatic communication means, said supply reservoirs and said brake cylinder devices. 
     
     
       4. A transducer calibration system as recited in claim 1, wherein said mathematical best fit curve is generated for each of said pneumatic communication means, said supply reservoir and said brake cylinder pressures for said train. 
     
     
       5. A transducer calibration system as recited in claim 1, wherein said best fit curve is generated reiteratively when the difference between said theoretical reference value and said transducer feedback signal corresponding thereto is greater than a predetermined percentage of said theoretical reference value. 
     
     
       6. A transducer calibration system as recited in claim 4, wherein said means for generating said best fit curve effects pressure equalization between said pneumatic communication means, said supply reservoir and said brake cylinder device on each said car when said pneumatic communication means is substantially charged, said equalization pressure at each said car providing the basis on which said best fit curve is generated according to a fourth order polynomial. 
     
     
       7. A transducer calibration system as recited in claim 6, further comprising: a. an application valve between said supply reservoir and said brake cylinder device having an open position in which fluid pressure communication therebetween is established when said pneumatic communication means is substantially charged; and   b. choke means between said pneumatic communication means and said supply reservoir for charging said supply reservoir to the pressure of said pneumatic communication means.   
     
     
       8. A transducer calibration system as recited in claim 7, further comprising means for sensing substantial pressure equalization between said supply reservoir and said brake cylinder device at a preselected car, and accordingly commanding a reduction of the pressure in said pneumatic communication means to within a predetermined value of said equalization pressure effective at said preselected car to provide pressure equalization between the reduced pressure of said pneumatic communication means, and the equalized pressure of said supply reservoir and said brake cylinder device. 
     
     
       9. A transducer calibration system as recited in claim 8, further comprising a charging valve between said pneumatic communication means and said supply reservoir in parallel with said choke means, said charging valve being operated to an open position following said pressure equalization between said pneumatic communication means, said supply reservoir and said brake cylinder device. 
     
     
       10. A transducer calibration system as recited in claim 6, further comprising a release valve having an open position in which said brake cylinder device is vented to atmosphere, and said application valve having a closed position in which fluid pressure communication between said supply reservoir and said brake cylinder device is interrupted, during said charging of said pneumatic communication means, whereby said pressure transducer feedback signal corresponding to said brake cylinder device represents a zero pressure offset value. 
     
     
       11. A transducer calibration system as recited in claim 10, wherein said zero pressure offset value and said error correction factor for said brake cylinder pressure transducer are used to calculate a variable error correction factor according to the following linear equation: ##EQU2## where: C=pressure correction factor P R  =transducer pressure reading   C 0  =zero pressure offset   P T  =theoretical reference pressure   C 1  =pressure offset from P T .   
     
     
       12. A method of calibrating pressure transducers in an electro-pneumatic brake control system for a railroad train having pneumatic and electric communication means extending from the train locomotive through each car thereof, said locomotive and said cars having microprocessor means to which said electric communication means is connected, each said car further having a supply reservoir connected to said pneumatic communication means, a brake cylinder device connected to said supply reservoir and pressure transducers providing electric feedback signals to said car microprocessor means corresponding to the fluid pressure effective at said pneumatic communication means, said supply reservoir and said brake cylinder device, comprising the steps of: a. charging said pneumatic communication means;   b. connecting said supply reservoir with said pneumatic communication means;   c. establishing fluid pressure communication between said supply reservoir and said brake cylinder device prior to said pneumatic communication means being fully charged;   d. detecting at a preselected one of said cars substantial pressure equalization between said supply reservoir and said brake cylinder device thereof;   e. reducing the pressure of said pneumatic communication means to a value corresponding substantially to the equalization pressure of said supply reservoir and said brake cylinder device to obtain substantial pressure equalization therewith;   f. calculating from said transducer feedback signals effective at respective ones of said cars a best fit curve for at least one of said pneumatic communication means, said supply reservoir and said brake cylinder device, said best fit curve approximating the natural train pressure gradient therefor;   g. deriving from said best fit curve a theoretical reference value for each car depending on its location in said train; and   h. detecting a deviation between said theoretical reference value and said transducer feedback signal corresponding thereto for each said car to derive a transducer error correction factor.   
     
     
       13. The method as recited in claim 12, wherein said supply reservoir is connected with said pneumatic communication means via a choke. 
     
     
       14. The method as recited in claim 13, further comprising the step of connecting said supply reservoir with said pneumatic communication means in bypass of said choke following said reduction of the pressure of said pneumatic communication means when pressure equalization between said supply reservoir and said brake cylinder device is detected. 
     
     
       15. The method as recited in claim 12, further comprising the step of determining said substantial pressure equalization between said supply reservoir and said brake cylinder device in accordance with the difference between said feedback signals of said transducers corresponding thereto being less than a predetermined value. 
     
     
       16. The method as recited in claim 15, wherein said predetermined value is 0.5 psi. 
     
     
       17. The method as recited in claim 12, wherein the pressure of said pneumatic communication means is reduced to within a predetermined value of said equalization pressure effective at said supply reservoir. 
     
     
       18. The method as recited in claim 17, wherein said predetermined value is 1.0 psi. 
     
     
       19. The method as recited in claim 12, further comprising the steps of: a. detecting a difference between said theoretical reference value and said transducer feedback signal corresponding thereto; and   b. reiterating steps (f), (g) and (h) of claim 10 disregarding any such pressure transducer feedback signal when the difference between said feedback signal and said corresponding theoretical reference value exceeds a predetermined amount.   
     
     
       20. The method as recited in claim 19, wherein said predetermined amount is 10 percent of said theoretical reference value. 
     
     
       21. The method as recited in claim 12, wherein said transducer error correction factor is constant for any pressure of said at least one of said pneumatic communication means, said supply reservoir and said brake cylinder device. 
     
     
       22. The method as recited in claim 12, wherein said at least one of said pneumatic communication means, said supply reservoir and said brake cylinder device is said brake cylinder device. 
     
     
       23. The method as recited in claim 22, further comprising the steps of: a. releasing fluid under pressure from said brake cylinder device during said charging of said pneumatic communication means prior to said fluid pressure communication being established between said supply reservoir and said brake cylinder device;   b. detecting substantially complete exhaust of said brake cylinder fluid under pressure;   c. providing a zero offset value according to the difference between said transducer feedback signal corresponding to said brake cylinder device and zero psi;   d. calculating a linear equation in accordance with said zero offset value and said error correction factor for said brake cylinder pressure transducer at each said car; and   e. deriving from said linear equation a variable error correction factor.   
     
     
       24. The method as recited in claim 23, wherein said linear equation is as follows: ##EQU3## where: C=pressure correction factor P R  =transducer pressure reading   C 0  =zero pressure offset   P T  =theoretical reference pressure   C 1  =pressure offset from P T .   
     
     
       25. The method as recited in claim 23, wherein said transducer error correction factor is constant for any pressure of said at least one of said pneumatic communication means and said supply reservoir. 
     
     
       26. The method as recited in claim 22, further comprising the steps of: a. providing a predetermined delay period following commencement of said release of fluid under pressure from said brake cylinder device;   b. monitoring each said car brake cylinder pressure following expiration of said delay period; and   c. determining a faulty brake system when said brake cylinder pressure is greater than a predetermined critical value.